Iron and iron metal containing alloys such as mild steel are well-known materials used in constructing the apparatus of aqueous systems in which system water circulates, contacts the iron based metal surface, and may be concentrated, such as by evaporation of a portion of the water from the system. Even though such metals are readily subject to corrosion in such environments, they are used over other metals due to their strength and availability.
It is known that various materials which are naturally or synthetically occurring in the aqueous systems, especially systems using water derived from natural resources such as seawater, rivers, lakes and the like, attack iron based metals (the term "iron based metals" shall mean in the present disclosure and the appended claims iron metal and metal alloys containing iron therein, i.e. ferrous metals). Typical devices in which the iron metal parts are subject to corrosion include evaporators, single and multi-pass heat exchangers, cooling towers, and associated equipment and the like. As the system water passes through or over the device, a portion of the system water evaporates causing a concentration of the dissolved materials contained in the system. These materials approach and reach a concentration at which they may cause severe pitting and corrosion which eventually requires replacement of the metal parts. Various corrosion inhibitors have been previously used.
Chromates and inorganic polyphosphates have been used in the past to inhibit the corrosion of metals which is experienced when the metals are brought into contact with water. The chromates, though effective, are highly toxic and, consequently, present handling and disposal problems The polyphosphates are relatively non-toxic, but tend to hydrolyze to form orthophosphate which in turn can create scale and sludge problems in aqueous systems. Moreover, where there is concern over eutrophication of receiving waters, excess phosphate compounds can provide disposal problems as nutrient sources Borates, nitrates, and nitrites have also been used for corrosion inhibition. These too can serve as nutrients in low concentrations, and represent potential health concerns at high concentrations. In addition, environmental considerations have also recently increased concerns over the discharge o other metals such as zinc, which previously were considered acceptable for water treatment.
Much recent research has concerned development of organic corrosion inhibitors which can reduce reliance on the traditional inorganic inhibitors. Among the organic inhibitors successfully employed are numerous organic phosphonates. These compounds may generally be used without detrimental interference from other conventional water treatment additives.
Phosphonic acid compounds have also been used in other fields for such purposes as flame retardants, plasticisers, lubricants, and surfactants. U.S. Pat. No. 3,032,578 discloses, for example, certain aryloxypolyalkylene ether phosphonates which exhibit useful properties as surface active agents, and are prepared by reacting a trialkyl phosphite with an aryloxypolyalkylene ether halide to form the corresponding dialkyl phosphonate, which is hydrolyzed to the corresponding phosphonic acid.